1. Field of the Invention
This invention relates to a biological signal transmission apparatus of a medical telemeter for transmitting a biological signal from a transmitter through an antenna to a receiver and in particular to a biological signal transmission apparatus using a loop antenna, a microstrip antenna as an antenna.
2. Related Art
A system for transmitting by radio a biological signal detected on an electrode placed on a subject to a nearby computer diagnostic apparatus, etc., via an antenna for diagnosis is known. Hitherto, various propositions have been made as a transmission apparatus used with such a system.
In a proposition described in JP-A-60-97103U, two electrodes 502 and 503 attached to a chest belt 501 and a transmitter main unit 504 placed on a wrist of a subject are connected by electrode leads 505 and 506, as shown in FIG. 34. An antenna line 507 from the transmitter main unit 504 is placed closely on the leads 505 and 506 in parallel therewith and an end of the antenna line 507 is buried in the chest belt 501. The electrode leads 505 and 506 and the antenna line 507 are insulated from each other and the end of the antenna line 507 is also electrically insulated so as not to touch the body surface of the subject.
According to the proposition, the antenna line 507 is placed closely on the leads 505 and 506 and thus can be made 1 m or longer without disturbing any motion, and the efficiency of the transmitter 504 can be improved and miniaturized for enhancing portability of the transmitter.
In a proposition described in JP-A-62-202804U, a pair of electrodes 201 and 202 is placed in unit cases 203 and 204, which are opened at bottoms for exposing the electrodes 201 and 202, and both ends of an antenna line 205 are connected to the electrodes 201 and 202, as shown in FIG. 35. The unit cases 203 and 204 are coupled by a connection cable 206 and the antenna line 205 is inserted into the connection cable 206.
According to the proposition, the electrodes 201 and 202 placed in a pair of unit cases 203 and 204 are fitted to a heart rate detection part of a living body and a signal from the antenna line 205 is transmitted, so that the device is easily attached and detached and moreover can be placed without an oppressive feeling or a feeling of wrongness on the chest of the subject.
In a proposition described in JP-A-63-32501U, a device comprises a pair of electrodes 301 and 302, a transmitter main unit 303 having electric circuitry for processing an electrocardiographic signal detected on the electrodes 301 and 302, and an antenna 304 for sending the resultant signal to a receiver by a radio wave, as shown in FIG. 36. The antenna 304 is covered with water-repellent fibers and is put on the surface of a human body.
According to the proposition, the antenna 304 is covered with water-repellent fibers and is connected to the transmitter main unit 303 so that it is put on the surface of a human body. Thus, when the device is attached to a subject, clothes of the subject do not swell locally and moreover it is not feared that the electrode 301, 302 may be off the attachment point. Resultantly, sufficiently strong radio waves can be sent to the receiver in addition to ease of use.
In a proposition described in JP-A-9-108194, a base sheet 401 placed on the anterior chest wall of a subject is formed like an L letter, a longwise portion 401a is put along the breast bone line of the subject, and a widthwise portion 401c is directed toward the heart side from a corner 401b positioned near the xiphisternum of the subject, as shown in FIG. 37. The base sheet 401 is formed on a rear with an adhesion layer made to adhere to the anterior chest wall. A first electrode 402 is attached in the proximity of the corner 401b, a second electrode 403 is attached in the proximity of the upper end part of the longwise portion 401a, and a third electrode 404 is attached in the proximity of a side end part of the widthwise portion 401c. Further, a fourth electrode 405 is attached slantingly below the second electrode 403 and a fifth electrode 406 is attached above the third electrode 404.
Of the five electrodes arranged as described above, α induction is detected between the electrodes 402 and 403 and β induction is detected between the electrodes 403 and 404. γ induction for ischaemia of side and front and rear walls in a high-potential direction weak in sensitivity only with α induction and β induction is detected by means of the electrodes 405 and 406. The electrocardiographic signals induced to the electrodes are amplified and modulated by a circuit unit 407 attached to the base sheet 401 and are transmitted to the receiver through an antenna 408 attached along the longwise portion 401a. 
According to the proposition, the electrodes 402 to 406, the circuit unit 407, and the antenna 408 are mounted integrally on the base sheet 401, so that the device is easily placed on the subject and action is not limited.
In the examples in the related arts described above, the antennas are monopole antennas using the electricity length of a quarter the wave length. For example, assuming that the transmission frequency is 300 MHz, the wave length is 1 m and the antenna length becomes 25 cm. To place the monopole antenna so that it is not affected by a human body as much as possible, the monopole antenna may be placed in a direction perpendicular to the surface of a human body and distant from the human body. However, the antenna length is long (in this case, 25 cm), thus when the transmitter is placed on a human body, it disturbs the motion of the human body. If the transmitter is placed along the surface of the human body so as to facilitate the motion, radio waves radiated from the antenna are affected by the human body as described above, thus the gain is easily degraded. Also, although employing small and compact transmitter and electrode, there is still a problem that long using for standard limb lead (II) between electrodes disturb patient.